Reservoir simulation is an area of reservoir engineering that employs computer models to predict the transport of fluids, such as petroleum, water, and gas, within a reservoir. Reservoir simulators are used by petroleum producers in determining how best to develop new fields, as well as generate production forecasts on which investment decisions can be based in connection with developed fields.
Reservoir simulation software models are typically implemented using a number of discretized blocks, referred to interchangeably herein as “blocks,” “grid blocks,” or “cells.” Models can vary in size from a few blocks to hundreds of millions of blocks. In these software simulations, it is common to model a reservoir using a grid formed of grid blocks and then simulate reservoir properties (e.g., pressure, temperature) within each grid block to predict flow. For example, such modeling is particularly useful in low permeability reservoirs for determining how many and where fractures should be induced in a reservoir to achieve a certain flow over a period of time.
In any event, the application of grid blocks in a reservoir simulation is very dependent on the type of reservoir being simulated. Typically, shale reservoirs have extremely low permeability when compared to other types of geologic reservoirs. For example, shale reservoirs may be less permeable than other geologic reservoirs by a factor of 10−6. As a result, flow in a portions of shale reservoir must be modeled at a very fine grid scale when compared to non-shale reservoirs, which may often be modeled with coarser grids, i.e., grid with larger grid blocks. Because fine grid scales are computationally undesirable over a large area, it is common in the industry to apply fine grids only to local areas of interest, such as around a fracture, and apply a coarser grid across the remainder of the reservoir. These fine grids around local areas of interest are referred to as local grid refinements (“LGRs”) and are typically used to envelope the estimated region of the reservoir in which significant fluid and/or pressure movement will occur. Heretofore, for extremely low permeability reservoirs, there has never been a systematic process to determine the extent of this region of interest and where the boundary between LGRs and coarse grid blocks should exist. In other words, there is currently not a method in the prior art to a priori determine how far out from a fracture or other structure of interest to extent the fine grid blocks.